Advances in silicon processing have enabled integration of complex systems on a single low power chip. The low cost and low power consumption of such systems have resulted in proliferation of portable electronic devices. To operate, such devices must be frequently plugged into an electrical outlet to be charged.
Wireless power transmission may be achieved using inductive coupling or electromagnetic waves. Inductive coupling can deliver power over a short range. Electromagnetic (EM) waves may be used to transmit power over a longer distance. Both inductive coupling and EM waves cause an alternating current (AC) to be generated at the receiver. The AC current is subsequently converted to a direct current (DC). A higher frequency of operation may be used to reduce the size of a wireless power transmission recovery circuit. However, conventional diode rectifiers used in conventional electronic systems, such as power adapters and chargers, are not suited for operation at high frequencies.
FIG. 1 is a schematic diagram of a conventional rectifying circuit 10 adapted to convert an electromagnetic wave received via antenna 20 to a DC voltage across terminals OUT+ and OUT−. Circuit 10 is shown as including a matching LC network 12, a Schottky diode 14 and a RF bypass network 16. During one-half of each cycle when the anode terminal A of diode 14 has a higher potential than its cathode terminal B, diode 14 become conductive thus causing the received energy to be stored in electric and magnetic fields inside the matching network 12. During the other half of each cycle, diode 14 becomes non-conductive and the stored energy in matching network 12 as well as the incident energy are supplied across output terminals OUT+ and OUT−. Schottky diode 14 is required to operate in the GHz frequency range. While Schottky diode based rectifying circuits are efficient, they are not suitable for integration with CMOS technologies.
FIG. 2 is a schematic diagram of a conventional CMOS recovery circuit 50 driving load 72. Recovery circuit is shown as including NMOS transistors 52, 54, PMOS transistors 62, 64, and capacitor 70. Differential voltage VRF, delivered via node N1 by an antenna (not shown) receiving the incident EM wave is applied to the gate terminals of transistors 54, 64 and to source terminals of transistors 52, 62. Likewise, differential voltage VRF  delivered via node N2 by the antenna is applied to the gate terminals of transistors 52, 62 and to source terminals of transistors 54, 64. During one half of each cycle when differential voltage VRF is at a high voltage relative to the differential voltage VRF, transistors 52 and 64 are off and transistors 54 and 62 are on, accordingly a current flows from drain terminal of transistor 54 (ground terminal GND) to node N2, and from node N1 to output node OUT of load 72. During the other half of each cycle when differential voltage VRF is at a low voltage relative to the differential voltage VRF, transistors 52 and 64 are on and transistors 54 and 62 are off, accordingly a current flows from ground terminal GND to node N1, and from node N2 to output node OUT. In other words, during both cycles, the current flows out of the ground terminal GND and into the output node OUT thus rectifying the current.
Recovery circuit 50 is efficient in low power applications (such as RFID), but is not suitable for use in charging portable consumer electronics that require a relatively high power rectification. A need continues to exist for a rectifying circuit that is efficient and is adaptive to handle relatively higher power.